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The secret to it all is their aquatic environment. The average blue whale
for example is 85 feet long and weighs 106 tons, which is as much as 4
Brontosauri or 30 elephants. Other large baleen whales such as fin and
sei whales are much smaller than blue whales (tipping the scales on average
at 58 tons and 13 tons respectively),but they are still considerably larger
than the largest living terrestrial animal, the elephant. The reason whales
are able to support such weight is the buoyancy of the water in which they
live.

The heavier an animal is the greater its relative surface area. The power
of the legs and muscles which help support an animal is a function of the
surface of their cross-sections. As the total weight of an animal increases
a point is reached where the legs would just collapse. So once a certain
maximum weight is reached, life on land becomes impossible. The situation
is quite different in water however, where the buoyancy counteracts the
gravitational pull on the body. But note that even aquatic animals have
some limits on the size they can attain, for the surface area of the lungs,
intestines, red blood cells and kidneys becomes relatively smaller with
increase in total weight. This means that above a certain point the organs
would not be able to handle the metabolic requirements of such a gargantuan
body, but the problems of food supply would probably set a limit on further
growth long before that point is reached.

Comment

Isn't this a little bit like asking why ants are so small? Not poking fun but when you look at the massive diversity of life on earth there's bound to be something at some point in time which just happens to be much bigger than everything else! There are many unanswerable questions, I mean, why do kangaroos bounce along like springs when everything else seems to get along much better on four legs? If insects can avoid flying into most things then why do they have such a problem with cars? Maybe its the highly reflective surface???